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Message-ID: <20150928075851.GA23998@gmail.com>
Date: Mon, 28 Sep 2015 09:58:51 +0200
From: Ingo Molnar <mingo@...nel.org>
To: Denys Vlasenko <dvlasenk@...hat.com>
Cc: "H. Peter Anvin" <hpa@...or.com>,
Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
Boris Ostrovsky <boris.ostrovsky@...cle.com>,
David Vrabel <david.vrabel@...rix.com>,
Joerg Roedel <joro@...tes.org>, Gleb Natapov <gleb@...nel.org>,
Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
x86@...nel.org, linux-kernel@...r.kernel.org,
Andy Lutomirski <luto@...nel.org>,
Brian Gerst <brgerst@...il.com>,
Thomas Gleixner <tglx@...utronix.de>,
Borislav Petkov <bp@...en8.de>,
Peter Zijlstra <a.p.zijlstra@...llo.nl>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH] x86: Use entire page for the per-cpu GDT only if
paravirt-enabled
* Denys Vlasenko <dvlasenk@...hat.com> wrote:
> On 09/26/2015 09:50 PM, H. Peter Anvin wrote:
> > NAK. We really should map the GDT read-only on all 64 bit systems,
> > since we can't hide the address from SLDT. Same with the IDT.
>
> Sorry, I don't understand your point.
So the problem is that right now the SGDT instruction (which is unprivileged)
leaks the real address of the kernel image:
fomalhaut:~> ./sgdt
SGDT: ffff88303fd89000 / 007f
that 'ffff88303fd89000' is a kernel address.
fomalhaut:~> cat sgdt.c
#include <stdio.h>
#include <stdlib.h>
int main(void)
{
struct gdt_desc {
unsigned short limit;
unsigned long addr;
} __attribute__((packed)) gdt_desc = { -1, -1 };
asm volatile("sgdt %0": "=m" (gdt_desc));
printf("SGDT: %016lx / %04x\n", gdt_desc.addr, gdt_desc.limit);
return 0;
}
Your observation in the changelog and your patch:
> >> It is page-sized because of paravirt. [...]
... conflicts with the intention to mark (remap) the primary GDT address read-only
on native kernels as well.
So what we should do instead is to use the page alignment properly and remap the
GDT to a read-only location, and load that one.
This would have a couple of advantages:
- This would give kernel address randomization more teeth on x86.
- An additional advantage would be that rootkits overwriting the GDT would have
a bit more work to do.
- A third advantage would be that for NUMA systems we could 'mirror' the GDT into
node-local memory and load those. This makes GDT load cache-misses a bit less
expensive.
The IDT is already remapped:
fomalhaut:~> ./sidt
Sidt: ffffffffff57b000 / 0fff
fomalhaut:~> cat sidt.c
#include <stdio.h>
#include <stdlib.h>
int main(void)
{
struct idt_desc {
unsigned short limit;
unsigned long addr;
} __attribute__((packed)) idt_desc = { -1, -1 };
asm volatile("sidt %0": "=m" (idt_desc));
printf("Sidt: %016lx / %04x\n", idt_desc.addr, idt_desc.limit);
return 0;
}
Thanks,
Ingo
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